The present invention relates to an ignition (ignitor) apparatus and, more particularly, to a plasma jet ignition apparatus for generating and discharging a jet of plasma for igniting fuel in combustion chambers of power sources such as internal combustion engines and the like.
Power sources such as internal combustion engines and the like rely on the combustion of fuel as their primary source of energy. This combustion usually occurs in one or more combustion chambers where the fuel is ignited by various ignition (ignitor) means (ignition by compression is normally used in diesel engines). The fuel that is most often used for these power sources are hydrocarbon based fuels such as gasoline and diesel fuel. In the past, the efficiency of the power sources which used these fuels was not as important as it is today. World events have made the crude oil from which many of those fuels are derived both expensive and in scarce supply. The need for refinement of that crude oil into various octane and cetane levels required for conventional ignition systems also adds to this expense. Additionally, environmental concerns have required that such power sources provide for improved efficiency and reduced emission of environmentally harmful exhaust by-products. This present situation calls for higher efficiency engines and less costly fuels.
Efficiency can be improved and emissions reduced by the use of special materials and structural changes to recuperate kinetic and thermal energies from the exhaust gases. Many attempts and designs have been directed to this aspect of the problem. The other means for improving efficiency and reducing emissions is by the improvement of the combustion process which can be accomplished through the use of efficient ignition apparatus for the combustion of lean air/fuel mixtures. Present ignition systems (in premixed engines) use conventional spark plugs which discharge a high voltage-low energy spark of approximately 0.01 Joules into the combustible mixture. This spark ignites a small volume of the mixture which in turn spreads through the volume of the mixture at the speed of the flame front to ignite the rest of the mixture. This mixture usually contains a high-octane gasoline fuel in a rich air/fuel mixture. Lean air/fuel mixltures do not burn as well because the flame speed of the front is reduced. Because the actual burning rate and ignition delay of this system depends upon the physical chemistry of the fuel extremely sophisticated combustion chambers have been necessary to produce slight improvements in ignition delay and burning rate. Additionally, high octane or high cetane fuels are needed which are more expensive and scarce. Moreover, because conventional spark plug ignition systems require relatively rich air/fuel mixtures for proper combustion it is important to precisely maintain this air/fuel mixture for efficient operation. Thus the conventional ignition systems limit the useful operating range of both low and high compression ratio internal combustion engines and the like.
An ignition system that could reduce the fuel ignition delay and promote faster burning rates would improve fuel economy, reduce emissions, and extend the useful operating range of the engine in terms of the air/fuel mixture and also in terms of the types of fuels that could be used. Running an engine with lean air/fuel mixtures presents numerous of the above advantages. The excess air provides for nearly complete combustion of hydrocarbons and carbon monoxide which are usually released as exhaust gases. The greater dilution of the charge with a lean mixture results in a lower peak temperature attained within the combustion chamber. This determines lower heat loss and reduces the formation of nitric oxide pollutants. The ratio of specific heats of fuel-air mixtures increases as leaner mixtures are employed. This means higher thermal efficiency at a given compression ratio. Output power may be controlled by just the variation of the air/fuel ratio in a lean mixture. This avoids the use of a throttle valve, which generally introduces pressure drops and a resulting decrease efficiency. Thus the use of lean mixtures results in a decrease in pollutant production and increases in efficiency.
As pointed out, conventional spark plugs do not efficiently cause combustion of lean mixtures and either misfiring occurs or there is no combustion. The typical spark of a conventional spark plug is highly localized and ignites a very small volume of fuel in the general vicinity of the surface of the spark. The small initial flame front produced from the spark propagates at a speed that is a function of the air/fuel ratio and of the chemical property of the fuel. With lean air/fuel ratios the combustion chemical kinetics is much slower. For efficient burning of such mixtures the flame speed must be increased.
The stratified charge engine is one structure which has been employed to attempt to gain the benefits of burning leaner air/fuel mixtures. The basis of this design is to provide for an initial combustion chamber in which a very rich air/fuel mixture is first ignited into a flame. Because of the pressure resulting from the chemical combustion this flame then enters the main combustion chamber to ignite a leaner mixture contained within the main chamber. The process requires chemical combustion in the initial chamber and the restructuring of the basic design of various internal combustion engines so that this initial chamber is provided for. These engines also require additional components, valves, and other design changes to present engines to allow for the use of the initial combustion of a rich mixture.
Another system for burning lean mixtures is based on the use of plasma jets. Basically these various systems create a jet of plasma which is introduced into the main combustion chamber. This jet causes the combustion of the fuel in the combustion chamber. The basic structure provides for an initial cavity in which a small amount of gas or the like is introduced. This gas is subjected to an electric discharge of high energy. This causes the gas to become a hot partially ionized gas otherwise known as plasma. Because of a great and quick buildup in pressure this plasma rushes out of an orifice in the cavity into the main combustion chamber as a jet or plume of plasma. Unlike the stratified engine flame this jet contains a much higher concentration of active chemical species (OH, H, N . . . radicals), and enters the combustion chamber at supersonic speeds. It appears that the presence of the radicals and the induced small scale turbulence enhances the initiation and the propagation of the flame front in lean air/fuel mixtures.
It has been recognized in the art that a plasma jet ignition system would have many advantages for use in internal combustion engines. Plasma jet ignitors can be adapted to be placed into internal combustion engines with relative ease. They offer an excellent means by which lean mixtures may be burned to extend the operating ranges of conventional engines. This of course provides all of the advantages of burning of lean mixtures in terms of fuel savings and pollutant reduction.
As has been pointed out the plasma medium, the magnitude and duration of the energy that generates the plasma, the size and shape of the plasma cavity and the size and shape of the orifice all affect plasma jet ignition effectiveness. The initial velocity of the plasma jet as it enters the main combustion chamber governs the penetration of the jet and its ability to cause small scale turbulence and enhance combustion. This velocity has been controlled by the dimensions of the plasma forming cavity and the ejection orifice. The duration and the amount of energy imparted to the plasma also governs the initial velocity. Higher energies must be discharged through the spark plug electrodes than for conventional spark plugs to generate plasma jets of sufficient pressure to be able to achieve the advantages of penetration and turbulence mentioned for enhanced combustion. These high energies tend to erode electrodes at a faster rate and to erode the orifice and cavity shape of the plugs.
The present invention provides for a plasma jet ignitor that can be easily adapted for use with internal combustion engines. It improves combustion and reduces pollutants by providing a jet of plasma that will ignite lean levels of fuel/air mixtures. The invention also provides for an external magnetic field means to accelerate the plasma jet so that the jet achieves good initial velocity so that it achieves the appropriate penetration into the combustion chamber to provide for the most efficient combustion of the fuel mixture. Because of the use of external means to accelerate the jet, the initial energy needed for the electrode discharge does not need to be as great. This means that the ignitor durability will be increased and that the entire system will be part of an actual power plant. Further advantages and features of present invention are discernable from the disclosure that follows.